Corrosion damage is a significant threat to the safe operation of both military and commercial aircraft. Failure to detect and correctly quantify corrosion damage can lead to catastrophic failure of various aircraft components. Such failures can lead to the loss of the aircraft as well as loss of lives. Corrosion damage is an even more significant problem with military aircraft due to the extreme environments in which they must operate.
The relevant art is replete with systems for determining damage to metal structures. One such system includes a thermographic system for detecting exfoliation corrosion on aircraft skins. However, this technique is limited to flat surfaces and only presents images that are visually compared with surrounding areas. Traditional impedance plane eddy current techniques are also used to evaluate corrosion loss on aircraft skins. The eddy current response from a test sample is compared to the eddy current response from a reference standard. The result is a subjective evaluation of whether or not the test sample is better or worse than the reference standard. The technique is limited by the inability to produce corrosion reference standards with quantifiable defects. As a result, assessment of corrosion damage in an aircraft metallic structure has required a good understanding of the physics involved and extensive experience of a specialist in complex and time consuming inspection techniques currently available in the industry. In other words, corrosion damage determination has been highly subjective and quality depends to a large degree upon the individual conducting the evaluation.
Conventional eddy current inspection methods are typified by U.S. Pat. No. 5,510,709 to Hurley. This patent discloses an eddy current surface inspection array probe and a method for detecting cracks and flaws in aircraft skin metal immediately surrounding rivets without requiring the removal of rivets or manual scanning. The method comprises positioning a probe concentrically around the rivet. The sense coil pairs of the probe either provide a zero reading corresponding to no cracks or defects or a non-zero reading corresponding to one or more cracks or defects in the surface. Thus, this system is used to locate only two dimensional anomalies such as cracks in the surface around the rivet. Furthermore, the method described in this patent does not provide a quantified measurement of the metal loss on the surface around the rivet.
Another method of detecting surface corrosion is described in U.S. Pat. No. 5,491,409 to Flora et al. The Flora invention permits the detection of surface corrosion on metallic structures which are insulated by a coating or cover, or covered with marine growth. Excitation coils are wrapped around a magnetizing yoke which carries an alternating current in order to produce an alternating magnetic field by the magnetized yoke. The alternating magnetic fields induce an eddy current which runs through the metal component between the legs of a yoke and a pair of magnetic flux sensors are differentially connected beneath one or more of the excitation coils to sense a signal response. Thus, defects located on the surface can be detected. Nevertheless, Flora is only of interest and does not provide a method of quantifying the volume loss of these defects.
Moreover, the great majority of damage to structural members due to corrosion originates from corrosion damage within the fastener holes. However, none of the current inspection methods mentioned are applicable to evaluating corrosion damage in fastener holes.